The object of the invention is a method for the production of a polymethylmethacrylate bone cement, a bone cement kit for use in said method, and a bone cement that can be produced through said method.
Polymethylmethacrylate (PMMA) bone cements are based on the pioneering work of Sir Charnley. PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator (N,N-dimethyl-p-toluidine) dissolved therein. The powder component, also called bone cement powder, comprises one or more polymers that are produced through polymerisation, preferably suspension polymerisation, based on methylmethacrylate and co-monomers, such as styrene, methylacrylate or similar monomers, a radiopaque agent, and the initiator, dibenzoylperoxide. Mixing the powder component and the monomer component, swelling of the polymers of the powder component in the methylmethacrylate generates a dough that can be shaped plastically and is the actual bone cement. During the mixing of powder component and monomer component, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide while forming radicals. The radicals thus formed trigger the radical polymerisation of the methylmethacrylate. Upon advancing polymerisation of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies.
Polymethylmethacrylate bone cements can be mixed through manual mixing of the cement powder and of the monomer liquid in suitable mixing beakers with the aid of spatulas. This can lead to air bubbles being enclosed in the bone cement dough, which can have a negative effect on the mechanical properties of the cured bone cement.
A large number of vacuum cementing systems has been described for preventing air inclusions in bone cement dough of which the following shall be specified here for exemplary purposes: U.S. Pat. Nos. 6,033,105 A, 5,624,184 A, 4,671,263 A, 4,973,168 A, 5,100,241 A, WO 99/67015 A1, EP 1 020 167 A2, U.S. Pat. No. 5,586,821 A, EP 1 016 452 A2, DE 36 40 279 A1, WO 94/26403 A1, EP 1 005 901 A2, and U.S. Pat. No. 5,344,232 A.
Cementing systems, in which both the cement powder and the monomer liquid are already packed in separate compartments of the mixing systems and are mixed with each other in the cementing system only right before application of the cement, are a development of cementing technology. Closed Full-Prepack mixing systems of this type have been described in patents EP 0 692 229 A1, DE 10 2009 031178 B3, U.S. Pat. No. 5,997,544 A, WO 00/35506 A1, and U.S. Pat. No. 5,588,745 A.
The use of virtually all known full-prepack mixing systems requires the medical user to manually mix the cement components, the cement powder and the monomer liquid by actuating mixing devices, such as mixing rods with stirring vanes, to form a cement dough. The homogeneity of the cement dough thus formed depends significantly on the procedure of manual mixing and can therefore be subject to variation.
Mixing methods and facilities that do not require manual mixing of the components, as are described in WO 00/35506 A1 and U.S. Pat. No. 5,588,745 A, require facilities of a complicated design and/or often work only under special prerequisites.
Accordingly, to prevent possible inhomogeneities of the cement dough, WO 00/35506 A1 proposed a storage and mixing device, in which the cement components are mixed with each other in the absence of a manually-driven mechanical mixing process. In this context, the polymethylmethacrylate bone cement powder is stored in a cartridge, whereby the cement powder takes up the entire volume of the cartridge and the volume of the intervening spaces between the particles of the cement powder is equal to the volume of the monomer liquid required for the production of bone cement dough using the cement powder stored in the cartridge. The design of this device is such that the monomer liquid is guided into the cartridge from above by the action of a vacuum and is drawn through the cement powder by a vacuum that is applied to a vacuum connector on the underside of the cartridge, whereby the air situated in the intervening spaces of the cement particles is displaced by the monomer liquid. This involves no mechanical mixing of the cement dough thus formed by means of a stirrer. It is a disadvantage of this system that the currently commercially available cement powders cannot be mixed with said device, because the rapidly swelling cement powder particles form a gel-like barrier after ingress of the monomer liquid into the cement powder to a depth of approximately 1-2 cm and impede the further migration of the monomer liquid through the entire cement powder. Moreover, conventional cement powders show a phenomenon, which is that the powder particles are wetted only poorly by methylmethacrylate due to the different surface energies. As a result, the methylmethacrylate penetrates only slowly into the cement powder.